So, if we are focusing back on the tiling problem, I want to remind that the more serious possible issue is not the one that is so obvious to confirm like shown above "rabbit planet" with a lot of the same "stamp" on the same planet.
If I understood "If any patch of land is known to repeat itself, even if it's only just once per planet," correctly, Fnool was refering to usage of the same pattern on SEVERAL planets, what is would be more concerning IMO.
ATM I can think of the infamous rayed-crated, and a "zig-zag" terrain feature I have pictured on different planets and already pasted here myself.
Of such kind of repetition happens on larger scale then we managed to find out until now, it would be indeed a design flaw with fatal consequences to the very sense of exploring.